Creating secure self signed certs in golang

main.go

package main

import (
	"crypto/rand"
	"crypto/rsa"
	"crypto/tls"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/pem"
	"errors"
	"log"
	"math/big"
	"time"
)

// CertTemplate is a helper function to create a cert template with a serial number and other required fields
func CertTemplate() (*x509.Certificate, error) {
	// generate a random serial number (a real cert authority would have some logic behind this)
	serialNumberLimit := new(big.Int).Lsh(big.NewInt(1), 128)
	serialNumber, err := rand.Int(rand.Reader, serialNumberLimit)
	if err != nil {
		return nil, errors.New("failed to generate serial number: " + err.Error())
	}

	tmpl := x509.Certificate{
		SerialNumber:          serialNumber,
		Subject:               pkix.Name{Organization: []string{"JaegerTracing"}},
		SignatureAlgorithm:    x509.SHA256WithRSA,
		NotBefore:             time.Now(),
		NotAfter:              time.Now().Add(time.Hour * 24 * 30), // valid for a month
		BasicConstraintsValid: true,
	}
	return &tmpl, nil
}

// CreateCert invokes x509.CreateCertificate and returns it in the x509.Certificate format
func CreateCert(template, parent *x509.Certificate, pub interface{}, parentPriv interface{}) (
	cert *x509.Certificate, certPEM []byte, err error) {

	certDER, err := x509.CreateCertificate(rand.Reader, template, parent, pub, parentPriv)
	if err != nil {
		return
	}
	// parse the resulting certificate so we can use it again
	cert, err = x509.ParseCertificate(certDER)
	if err != nil {
		return
	}
	// PEM encode the certificate (this is a standard TLS encoding)
	b := pem.Block{Type: "CERTIFICATE", Bytes: certDER}
	certPEM = pem.EncodeToMemory(&b)
	return
}

// NewTLSCredentials creates signed certificates for both the client and server
func NewTLSCredentials() (*x509.Certificate, tls.Certificate, []byte, tls.Certificate, []byte) {
	// generate a new key-pair
	rootKey, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		log.Fatalf("generating random key: %v", err)
	}

	rootCertTmpl, err := CertTemplate()
	if err != nil {
		log.Fatalf("creating cert template: %v", err)
	}

	// this cert will be the CA that we will use to sign the server cert
	rootCertTmpl.IsCA = true
	// describe what the certificate will be used for
	rootCertTmpl.KeyUsage = x509.KeyUsageCertSign | x509.KeyUsageDigitalSignature
	rootCertTmpl.ExtKeyUsage = []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth, x509.ExtKeyUsageClientAuth}

	rootCert, _, err := CreateCert(rootCertTmpl, rootCertTmpl, &rootKey.PublicKey, rootKey)
	if err != nil {
		log.Fatalf("error creating cert: %v", err)
	}

	/*******************************************************************
	Server Cert
	*******************************************************************/

	// create a key-pair for the server
	servKey, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		log.Fatalf("generating random key: %v", err)
	}

	// create a template for the server
	servCertTmpl, err := CertTemplate()
	if err != nil {
		log.Fatalf("creating cert template: %v", err)
	}
	servCertTmpl.KeyUsage = x509.KeyUsageDigitalSignature
	servCertTmpl.ExtKeyUsage = []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth}

	// create a certificate which wraps the server's public key, sign it with the root private key
	_, servCertPEM, err := CreateCert(servCertTmpl, rootCert, &servKey.PublicKey, rootKey)
	if err != nil {
		log.Fatalf("error creating cert: %v", err)
	}

	// provide the private key and the cert
	servKeyPEM := pem.EncodeToMemory(&pem.Block{
		Type: "RSA PRIVATE KEY", Bytes: x509.MarshalPKCS1PrivateKey(servKey),
	})
	servTLSCert, err := tls.X509KeyPair(servCertPEM, servKeyPEM)
	if err != nil {
		log.Fatalf("invalid key pair: %v", err)
	}

	/*******************************************************************
	Client Cert
	*******************************************************************/

	// create a key-pair for the client
	clientKey, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		log.Fatalf("generating random key: %v", err)
	}

	// create a template for the client
	clientCertTmpl, err := CertTemplate()
	if err != nil {
		log.Fatalf("creating cert template: %v", err)
	}
	clientCertTmpl.KeyUsage = x509.KeyUsageDigitalSignature
	clientCertTmpl.ExtKeyUsage = []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth}

	// the root cert signs the cert by again providing its private key
	_, clientCertPEM, err := CreateCert(clientCertTmpl, rootCert, &clientKey.PublicKey, rootKey)
	if err != nil {
		log.Fatalf("error creating cert: %v", err)
	}

	// encode and load the cert and private key for the client
	clientKeyPEM := pem.EncodeToMemory(&pem.Block{
		Type: "RSA PRIVATE KEY", Bytes: x509.MarshalPKCS1PrivateKey(clientKey),
	})
	clientTLSCert, err := tls.X509KeyPair(clientCertPEM, clientKeyPEM)
	if err != nil {
		log.Fatalf("invalid key pair: %v", err)
	}

	// rootCert is the client CA as well as server CA, servTLSCert is for the server
	return rootCert, clientTLSCert, clientKeyPEM, servTLSCert, servKeyPEM
}